In a boiling water reactor (BWR), water as the coolant moderator typically enters the fuel assembly from a bottom portion, flows through the fuel assembly, and exits to an uppermost portion. As is typical for nuclear fuel assemblies in BWR's, elongated nuclear fuel rods having end caps at opposite ends are supported at their lower and upper ends between lower and upper tie plates, respectively. The tie plates provide the basic upper and lower structural elements for the fuel assembly in a configuration for interfacing with the reactor core support and for fuel handling. The tie plates also serve to maintain the fuel rod spacing at the ends of the fuel rods. Spacer grids are positioned between the upper and lower tie plates for retaining the fuel rods parallel to one another and with fixed spacing in a fixed geometry. As the coolant moderator flows upward through the fuel assembly, heat is transferred to the coolant moderator.
In order to accommodate the longitudinal (i.e. axial) expansion of the fuel rods during reactor operations, the restraining holes in the lower tie plate which receive the fuel rod end caps are sized so the fuel rod end caps when positioned in their corresponding restraining holes are free to move. Typically, a gap of about 2 to 6 mils exists between the outer wall of the end cap and the wall of the restraining hole. As the coolant moderator enters the fuel assembly and flows into the assembly and by the fuel rods, flow induced vibration at the lower portion of the fuel rods positioned in the restraining holes in the lower tie plate can occur leading to fuel rod fretting wear and subsequent fuel rod failure or premature withdrawal of fuel rods from the reactor.
It is typically desirable to increase the amount of power that can be obtained from a fuel assembly by increasing the reactor coolant flow. However, increasing the reactor coolant flow increases the flow induced vibrations and thus fuel rod fretting. Increased power from a fuel assembly could be obtained if the active length of the fuel rod could be increased which would increase the amount of fuel. However, the active length of a fuel rod is limited by the overall length of the fuel assembly and the required length of the non-fueled lower end cap at the lower end of the fuel rod.
Thus, it would be an advantage over the prior art design if one could obtain more power from a fuel assembly by increasing the length of the active portion of the fuel rod(s) without increasing the length of the fuel assembly and at the same time reduce flow induced vibrations, fuel rod fretting, and subsequent fuel rod failure.